This present invention relates generally to digital wireless communication systems, and more specifically, to the estimation of pulse shaping filters and compensation of signal distortion caused by the pulse-shaping filters in a wireless communication system.
Mobile wireless terminals, such as cellular telephones, use pulse-shaping filters to filter transmit and receive signals. A basic function of the transmit pulse-shaping filter is to limit the bandwidth of the transmitted signal, while the function of the receive pulse-shaping filter is to recover the transmitted signal by filtering out noise and interference.
Knowledge of the response of the pulse-shaping filter can be used to improve the capacity and coverage in certain wireless systems. For example, knowledge of the transmit and receive pulse-shaping filters may be used to cancel interference arising from a co-channel base station by aiding channel estimation and tracking. Additionally, knowledge of the pulse-shaping filters may be used in a GSM (Global System for Mobile Communications) receiver to provide improved channel estimation for MLSE (maximum likelihood sequence estimation) equalization.
Knowledge of the pulse-shaping filters is encompassed in the impulse response. An impulse response is a mathematical function that describes the output waveform that results when the input is excited by a unit impulse function. The impulse response of the cascade of all transmit and receive pulse-shaping filters in a communication system, or the pulse-shape response, can be used to describe the response characteristics of the pulse-shaping filters. In a communication system using partial response signaling (e.g., GSM), the over-all channel impulse response, which is used by demodulators in the wireless terminals to recover the intended signal, consists of the impulse response to the transmission medium (i.e., the medium response) convolved with the pulse-shape response. The over-all channel response for fractional equalization for a full-response signaling system (e.g., the D-AMPS system) can also be broken down into the medium response and the pulse-shape response. Moreover, the channel response for a co-channel interferer not time-aligned with the desired signal, which is used for joint demodulation in an D-AMPS terminal, is also given by the convolution of the interferer's medium response and the pulse-shape response. Estimating the medium response with knowledge of the pulse-shape response is superior to estimating the over-all channel response. This is applicable in all the above-mentioned examples.
The pulse-shape response is largely fixed when the wireless terminal is manufactured. However, due to manufacturing and component variability, the pulse-shape response may not perfectly match the intended pulse-shape response. Additionally, an analog receive filter is susceptible to temperature variation and aging which may cause it to change in time. Distortion in the pulse-shape response may cause inter-symbol interference (ISI) and/or adjacent channel interference (ACI). Moreover, the disparity between the assumed and actual receive filters may lead to a performance loss in the above discussed techniques that rely on the pulse-shape response.